Traditionally, paper handling devices of the type including xerographic reproduction machines have incorporated some sort of registration system to properly align sheets of paper passing through these devices. Whether the sheet is a document in a recirculating document handler or a copy sheet in the reproduction processor, registration or alignment of the sheets traveling through a paper path to a known orientation is necessary for the achievement of high quality copying. With particular reference to the reproduction processor, it will be appreciated that registration of copy sheets must include, for example, synchronization of the copy sheet edges with the latent image on the photoreceptor, as well as speed matching with the photoreceptor and transportation of the sheet into the transfer zone (herein defined as registration in the process direction or registration in the X-direction), side registration (herein defined as registration in a direction perpendicular to the path of sheet travel or registration in the Y-direction), or deskewing of improperly fed copy sheets (herein defined as deskewing or registration in the .THETA. direction).
In the past, alignment systems have been primarily passive, making use of physical contact with the sheet for alignment generally by providing a fixed position contacting registration member at an appropriate position for a selected registration operation. Such contacting members have included gripper bars, side guides, tamper arrangements, stalled rolls and/or registration fingers. A common weakness in all these devices is the inherent relative motion between the registration member and the sheet during contact. Contact and slippage between registration member and sheet can cause unacceptable damage to the sheet edge, and potential jamming of the machine. In certain finger registration systems, misfeeding may lead to tearing, ripping or holing of the sheet, particularly problematic when handling originals. Additionally, long term contact between passing sheets and the registration member may cause wear of the registration member, leading to long term variance in registration, thereby requiring adjustment or repair of the mechanical members to obtain original registration accuracy. These arrangements often also require extended paper paths to be effective.
Use of active driving arrangements to alter the orientation of objects such as a flimsy sheet of paper, cardboard or cloth, by providing differentially driven driving members are known, as shown by, for example, U.S. Pat. Nos. 2,407,174 to Oberender; 3,758,104 to Dailey; 4,155,440 to Bogdanski et al; 3,131,931 to Fechowsky; 3,240,487 to Templeton; 3,897,945 to Faltot et al; 4,082,456 to Schroter; 4,500,086 to Garavuso; "Means to Correct Document Skew", Research Disclosure, November 1979, pp. 642-643, No. 18759; and West German Patent No. 1,028,945. Some of positive driving arrangements are provided with sensors to sense position of the objects, to deskew or position the objects, such as for example, U.S. Pat. Nos. 3,525,872 to Schneider; 4,082,456 to Schroter; 3,360,262 to Kekopoulos et al; Japanese Kokai 54-149175 and Japanese Kokai 52-20562, which teach detection of lead edge skew, and U.S. Pat. Nos. 1,951,901 to Cottrel, 3d; 3,368,726 to Funk et al; and 3,603,446 to Maxey et al; 3,883,134 to Shinaki which teach detection of side edge registration.
Certain registration systems provide active registration devices which sense document position and operate to correct the positioning if necessary. With particular reference to U.S. Pat. No. 4,438,917 to Janssen et al., a sheet deskewing arrangement may be provided with a sensor set arranged along the path of sheets in the processing direction and a pair of selectably controllable motors, each driving a driving nip in a nip roll pair, supported to contact a side portion of the sheet in driving engagement, to correct skew sensed by the sensors. The two sensors, arranged in a line perpendicular to the path of sheet travel, each detect when the lead edge of a sheet passes thereby. A difference in sensing time of sheet passage by each sensor is indicative of sheet skew, and the two motors are driven in accordance with the difference to accelerate or decelerate a side portion of the sheet, thereby rotating the sheet to bring the lead edge of the sheet into registration. This arrangement provides satisfactory registration, when paper is fed to the deskewing area with an induced initial skew angle, within a predetermined range of angles and in an appropriate direction. Additionally, this type of arrangement is attractive from the point of view that misregistration or misfunctioning of the registration apparatus results only in a misregistered copy, and does not damage the sheet or machine. U.S. Pat. No. 4,216,482 to Mason, teaches a combination of a hard stopping pivot member and a positive driving arrangement, coupled with fixed and movable sensors to register a sheet.